Smelting of metallurgical dusts



Patented Dee. e, 193s SMELTING F METALLURGICAL DUSTS Jesse 0. Bctterton, Metnchen, Albert J. Phillips,

Plainield, and Ross E. Allen, Avenel, N. J., assignors to American Smelting and Refining Company, New York, N. Y., a corporation of New Jersey Application August 28, 1937, Serial No. 161,448

v Claims. (Cl. 'l5-25) The present invention relates to the pyrometallurgical smeltlng of nely divided oxide materials of relatively easily volatilizable metals, such as, for example, zinc, `lead, and ktin-bearing dusts of the character pf so-called bag-house fume" which is evolved and collected in various metallurgical operations.

More specically, the present invention has for its principal object the provision of an improved process for smelting zinc, lead, and tin-bearing dusts, and the preparation of a charge thereof for smeltlng in a shaft furnace with collection of certain of the metals in the furnace, while the more volatile metals are collected in an external condenser, and with the venting of incondensible gaseous lreaction products from the furnace.

'I'he handling of such materials is difficult owing to the finely divided character of the dusts,

'which renders difficult the preparation thereof into a charge having suicient mechanical 'strength to support its own weight and at the same time presenting sufficient porosity and looseness to 'enable passage of gaseous reacticn products therethrough.

\ A further object of the present invention therefore lies 1n the provision of a method of preparation of the furnace charge so that when'introduced into the smeltlng furnace it will possess the necessary physical characteristics for satis-` factory smeltlng.

Further objects of the invention will become apparent as the description proceeds, and the f eatures of nvelty will be pointed out in particularity in the appended claims.

The process of the present invention resides importantly in the preparation of a charge of metallurgical fume, dust or other finely divided metallurgical materials to enable the same to be formed into a charge having requisite physical properties for smeltlng by any pyrornetallurgical smeltlng operation, the process being adapted to produce a charge which is especially suitable for smeltlng in a shaft furnace heated either by fuel or electrically; and accordingly, while the invention will be described specifically as applied to an electrothermic smeltlng operation, it Will be apparent that the inventionis not so limited, but that the following description is intended to be wholly illustrative in character with regard to the preparation of thecharge and the manner in which it is smelted'.A v

c Therefore, vthe invention will be understood more particularly; :with 'reference i tc the accompanying: drawing which shows, rather diagrammatically, a suitable furnace construction for practicing the present invention, in which drawing Fig. 1 is a sectional elevation of the furnace, showing the construction thereof and arrangement of electrodes.

Fig. 2 is a view similar to Fig. 1, but taken at right angles to the view in Fig. 1, and illustrating a condenser and connections and mountings therefor for condensing and collecting the metals vaporized in the smeltlng furnace.

Fig. 3 is a plan view of the furnace shown in Figs. 1 and 2.

Referring more particularly to the drawing, the furnace comprises a metallic shell 5 enclosing suitable refractories comprising nre-brick 'l which l5 are built up so as to leave a central hole 9 which receives a refractory tube I I, which may be made suitably out of carborundum (silicon carbide).

The tube Il is somewhat smaller than the hole which receives it for enabling insertion of packing I3, this packing being suitably magnesite. A

further layer of loose packing i5 is inserted between the shell 5 and the fire-brick 1. A suitable material for this outer packing layer is found in practice to be kieselguhralthough it will be` understood that a Wide choice of refractories is afforded, and the furnace design is also open to substantial modication as to details of its construction.

The tube Il extends to a substantial height above the furnace and forms the shaft for receiving the furnace charge. It is closed at its bottom Vend, as is indicated at I1, and it, together with the remainder of the furnace structure, is provided with suitable apertures or ports for receiving various instrumentalities, as will be pointed out in greater detail hereinafter.

Thus, through ports I9 and 2| in the furnace, which are diametrically opposite one another, are inserted electrodes 23, 25. The electrodes are mounted in refractory electrically insulating sleeves 21, 29, `which may be made of Alundum reinforced, for mechanical strength, with carborundum or the like and `which contain asbestos packing-3|. The space around the sleeves 2l, 29 is filled with crushed magnesite 33, and the inner end of each sleeve is closed with suitable stop members 35, made from Alundum, or the like. The electrodes 23, 25 are provided with cooling coils 31, 39, which 'prevent' overheating of the 50 With a clean-out port 4I which communicates 55 with a silicon carbide (carborundum) tube 43 which is mounted in the supporting brickwork 45 for the furnace, the tube 43 sloping downwardly for discharge purposes, the materialcollecting at the bottom of the shaft Il being directed into the clean-out tube 43 by providing a slope 41 in the bottom of the shaft H, this slope being built up from suitable refractory material. In order to maintain this material in fluid condition, the supporting brick-work 45 is provided with a burner port 49 and a flamespace 5| extending around the bottom end of the shaft Il, a suitable burner (not shown) being adapted to be inserted in the port 49, the flame from the burner playing around the bottom end of the shaft II.

At a suitable location near the top of the furnace, a carborundum tube 53 is inserted in an opening 55 provided therefor in the shaft Il, the tube 53 leading condensible gases from the shaft il into the condenser 51. l

The conduit 53 is cemented to the shaft Il and is packed with loose asbestos-packing 59, while asbestos cement or the like 6| substantially encloses the condenser 51. The condenser 51 is vented 63 for enabling escape of uncondensible gases, s'uch as carbon monoxide, and an additional vent is provided for enabling the condenser to be heated. Metal m-ay be tapped from the condenser through a tap hole 61, and a. burner 69 is provided to maintain the collected metal above its melting point, so as to collect the metal as liquid metal.

In practice, the furnace is charged with coke to the electrode level to support the charge, and as the charge feeds between the electrodes it becomes heated by its resistance to the passage of current which is supplied by leads indicated at 1I, 13, 15, 11, the furnace being charged through the top of the shaft H.

Where the charge to be smelted is largely composed of nes, the handling of such materials is diflicult, -as well as is the production of a suitable charge, which possesses the requisites for satisfactory smelting.

The primary requirements for a satisfactory charge are shown by practice to be:

1. Sufficient mechanical strength for a shaft operation, that is the charge must support its weight and maintain sufficient looseness to enable self-feeding of the charge to the melting zone and relatively easy passage of gases and vapors upwardly through the charge.

2. Slag formation at the smelting zone. In this connection, it may be mentioned that slag formation substantially above the smelting zone forms accretions to such an extent that the operation is rendered impracticable.

3. An incorporated flux to produce a fluid slag. Segregation of separate flux additions in the furnace is to be avoided.

Considerable experimentation has been necessary to find how to make up a charge which will fulfill the above requirements. The dust being smelted may contain, for example, 50% to '70% zinc, along with appreciable amounts of tin and lead. with minor amounts of copper, iron, antimony, arsenic, cadmium, nickel, etc., practically all cf which are in the form of oxides.

In order to handle such material, it must be agglomerated, together with a suit-able flux and reducing agent in order to form the charge, and many trials have been made to form the same into a suitable agglomerate or clinker. With ordinary technique, such material cannot be made into a clinker, but merely a granular powder. Thus, sintering the dust mixed with coal and with or without admixture of fluxing agents such as soda ash, or-iron scale and lime, failed to produce a charge which possessed requisite properties; also among other uxing materials experimented with unsuccessfully were zinc chloride and sodium chloride; calcium chloride and sodium chloride; caustic soda; and in none of these instances was there produced a slag sufficiently liquid to be tapped and a charge which possessed sufficient motility to feed to the electrodes.

However, it was found that with admixture of borax in amounts of 1.25%, or more, With the dust, there is produced a solid clinker, the hardness and density of which depend on the amount of borax and the method of roasting. The charge of dust is mixed dry with the desired amount of borax and about 6%-8% carbonaceous material as reducing agent, then wet to the correct consistency and sintered. It is found in practice that 2.5% borax produces a commercially satisfactory sinter, while 5% is even more satisfactory as regards the properties of the sinter, but this amount is rather objectionable from cost standpoint and smaller amounts of borax, down to 1.25%, produce sinters of acceptable properties.

In carrying out the sintering operation, the charge having the proper composition is ignited on the surface in any convenient type of sintering apparatus, air being drawn downwardly through the charge using a suction equal to 6 to 8 inches of water. The time may vary from '7 minutes to 25 minutes, depending on the operation, thickness of the coke in the charge, etc., it being preferred to use a 5 inc h bed on the sintering machine. 'Ihe fines from the sinter cake may be screened.

In accordance with the above, 541 parts by weight of bag-house dust were mixed with 13.5 parts by weight of borax glass and 38 parts by weight of coke and pugged with water in a barrel mixer. a twelve inch Dwight and Lloyd sintering machine with a vacuum of 7 inches, an operating time of 21/2 hrs. at an approximate rate of 215 parts by Weight per hour. The resulting sinter was crushed and screened to form 1A inch to 1/2 inch mesh.

There were recovered 284.8 parts by weight of oversized sinter and 166.2 parts by weight of fines. The bag-house dust assayed Per cent Pb 14.72 Sn 7.13 Zn 52.74

The sinter produced assayed Per cent Pb 10.46 Sn 7.46 Zn 56.72

This sinter formed an open charge in the furnace which was easily maintained during the furnace operation. The sinter not only has sufficient mechanical strength but also does not tend to fuse and to bridge across the shaft above the smelting zone. A fluid slag is obtained with no evidence of accretions, while the charge is easily maintained open, with the formation of a fluid slag during the furnace operation.

The importance of an open charge becomes evident when it is to be noted that as the reduc- `The resultant charge was sintered on i gradual rate.

tion proceeds, the metal vapors must rise through the charge to the outlet while the non-condensible gases pass on all the way up through the shaft ligand out through the open end thereof; and obviously, the charge must be sufficiently open to enable the passage of the gaseous materials therethrough and also to be self-feeding to the smelting zone as the reduction proceeds. The following specific example gives an illustrative procedure for carrying out the process of the present invention:

The furnace was charged with lump coke to the electrode level, and the reactive charge of mixed sinter (278 parts by weight of borax sinter and 39 parts by weight of coke) admixed with 250 parts by weight of mixed agglomerates prepared by fusing converter dust containing principally oxides of zinc, lead and tin with soda-ash,

(2 50 parts by weight of agglomerates, 35 parts by weight of coke, 12.5 parts by weight of borax glass) was then introduced into the shaft Il at such an initial rate that each portion of the charge became incandescent before subsequent additions were made, the temperature being maintained, however, below the volatillzation point of zinc, until the charge reached a height greater than the outlet 55. When the charge has been introduced to such level, further additions may be made without regard to the incandescence of the entire amount of charge, it being suilicient to simply maintain the depth at such a` rate that shaft losses of zinc vapor will be prevented.

The charge assayed as follows:

As indicated above, the charge is added to the shaft at such a rate as will prevent shaft losses of zinc. vapor, the power being increased at a The mixed` sinter charge was added rst,vfol1owed by the agglomerates.

The operating data on the above charge were as follows:

'Time to fill shaft to condenser outlet From time to time the zinc was tapped from the condenser, the several portions thus collected being combined, remelted and sampled.

There was a total zinc recovery of 63.8 parto by Weight, assaying Pb0.022%, Sn 0.007%.

There was also recovered blue powderto the amount of 12.0 parts by weight, containing 'I 8 parts by weight of metallics, assaying Pb, trace; Sn 0.2%; -Zn 94.4%, and 4.2 parts by weigh-.t of pulp, assaying Pb trace; Sn trace; Zn 96.4%.

Slag and solder were also tapped from the shaft. I

Slag recovery amounted to 40 parts by weight, assaying Pb 2.82%; Sn 1.925%; Zn 16.65%.

This slag contains borax, and it may be reused in preparing the subsequent charge of sinter. l

' Solder recovery amounted to 62.3 parts by weight, assaying Pb 54.76%; Sn 25.13%; Zn 12.2%.

What is claimed is:

1. A process of smelting metallurgical dusts comprising oxides of lead, tin, and zinc, which comprises sintering the dusts in the presence of sufiicient borax to form a clinker and a substantially non-compacting smelting charge, charging successive portions of the resulting sinter' into an electrothermic shaft furnace in the presence of reducing agents, regulating the initial charging so that each portion of the charge becomes heated throughout to incandescence before further additions of charge are made, educting zinciferous vapors from the shaft, `and thereafter maintaining the charge sufficiently deep to avoid appreciable losses of zinc vapor, and conducting the same into an .external condenser, condensing the said vapors into liquid zinc, and recovering solder metal from the reaction zone in the shaft.

2. A process of smelting metallurgical dusts comprising oxideslof lead, tin, and zinc, which comprises sintering the dusts in the presence of suicient borax to form a clinker and a substa-ntially non-compacting smelting charge, smelting the said charge in a vertical shaft furnace in the presence of suflicient reducing agents to reduce the said oxides completely to metal and at` sufficiently high temperatures to volatilize the major portion of zinc with only minor amounts of lead and tin, conducting the zinc vapors into an external condenser, and recovering solder metal from the smelting Azone in the shaft, While permitting uncondensable gaseous products to y pass out from the furnace through the charge.

v3. A process of smelting metallurgical dusts comprising oxides of lead. tin, and zinc, which comprises sintering the dusts in the presence of suilicient borax to form a clinker and a substantially non-compacting smelting charge, charging the resulting sinter into a smelting shaft furnace in the presence of reducing agents and at vsuch rate as to preclude evolution of substantial amounts of zincy vapors from the charge, heating the charge to smelting temperatures and sufficiently highly to volatilize the major portion of the zinc i'n the charge while maintaining the charge suiliciently high in the shaft to prevent appreciable loss of 4zinc vapor, conducting the said zinc vapor into an external condenser, and

recovering solder metal from the smelting zone in the shaft. 1

.4. A process oi. smelting metallurgical dusts comprising oxides of` lead, tin, and lzinc, which comprises sintering the dusts in the presence of suiiicient borax to form a clinker and a substantially non-compacting smelting charge, smelting the said charge in a vertical shaft furnace in the presence of sufficient reducing agents to reduce the said oxides to metal and at sufficiently high temperatures to volatilize the major portion of the zinc with only minor amounts of lead and tin, with the production of a uid borax-containing slag and solder metal, conducting the zinc vapors into an external condenser, condensing the zinc to liquid metal, recovering the slag and solder metal from the furnace, and re-using the gases and vapors upwardly through the charge, regulating the initial charging so that each portion of the charge becomes heated just short of the volatilization temperature of zinc metal be fore further additions of charge are made, and thereafter, continuously maintaining the charge sufficiently deep to prevent appreciable loss of zinc vapor, conducting the zinc vapor into an external condenser, condensing the said vapor to liquid zinc, and tapping solder metal from the l0 furnace shaft.

JESSE 0. BETTERTON. ALBERT J. PHILLIPS. ROSS E. ALLEN. 

